In this particular case, there were user accounts, listings of items per user, calculators of various sort, multiple API integrations, and on and on. They understood by the end of the discussion, but seeing an image of something that looked complete was enough to trick their mind into thinking a lot of development work had occurred when, in fact, none had occurred. Only preliminary graphic design had occurred. This was earlier in my career. I typically use wire-frames or zoomed-in detail images now along with starting the discussion by letting them know that these are just graphic ideas, there has been no development yet, we are just at the stage that we want to be sure we are matching their vision.

Well, sure. If all you want is buttons.

But if you want reasonable portability of the interface across different devices, and scale, and connection quality there's more to do.

Even just getting an interface that responds cleanly to resizing can be trickier than it looks because what is important changes as aspect and scale change. How you present things may categorically change.

And this doesn't even start on talking about how to get the backend to where it matches the implied functionality of the front end.